Cover; The Search for Alien Life; Exclusive Online Issues; by Staff Editor; 1 Page(s)

Table of Contents; The Search for Alien Life; Exclusive Online Issues; by Staff Editor; 1 Page(s)

Where Are They?; The Search for Alien Life; Exclusive Online Issues; by Ian Crawford, sidebar by Andrew J. LePage; 6 Page(s)

How common are other civilizations in the universe? This question has fascinated humanity for centuries, and although we still have no definitive answer, a number of recent developments have brought it once again to the fore. Chief among these is the confirmation, after a long wait and several false starts, that planets exist outside our solar system.

Over the past five years more than three dozen stars like the sun have been found to have Jupiter-mass planets. And even though astronomers have found no Earth-like planets so far, we can now be fairly confident that they also will be plentiful. To the extent that planets are necessary for the origin and evolution of life, these exciting discoveries certainly augur well for the widely held view that life pervades the universe. This view is supported by advances in our understanding of the history of life on Earth, which have highlighted the speed with which life became established on this planet. The oldest direct evidence we have for life on Earth consists of fossilized bacteria in 3.5- billion-year-old rocks from Western Australia, announced in 1993 by J. William Schopf of the University of California at Los Angeles. These organisms were already quite advanced and must themselves have had a long evolutionary history. Thus, the actual origin of life, assuming it to be indigenous to Earth, must have occurred closer to four billion years ago.

Is There Life Elsewhere in the Universe?; The Search for Alien Life; Exclusive Online Issues; by Jill C Tarter and Christopher F. Chyba; 6 Page(s)

For 40 years, scientists have conducted searches for radio signals from an extraterrestrial technology, sent spacecraft to all but one of the planets in our solar system, and greatly expanded our knowledge of the conditions in which living things can survive. The public perception is that we have looked extensively for signs of life elsewhere. But in reality, we have hardly begun our search.

Assuming our current, comparatively robust space program continues, by 2050 we may finally know whether there is, or ever was, life elsewhere in our solar system. At a minimum we will have thoroughly explored the most likely candidates, something we cannot claim today. We will have discovered whether life dwells on Jupiter's moon Europa or on Mars. And we will have undertaken the systematic exobiological exploration of planetary systems around other stars, looking for traces of life in the spectra of planetary atmospheres. These surveys will be complimented by expanded searches for intelligent signals.

Profile: Jill C. Tarter, An Ear to the Stars; The Search for Alien Life; Exclusive Online Issues; by Naomi Lubick; 2 Page(s)

In a photograph hanging outside her office, Jill C. Tarter stands a head taller than Jodie Foster, the actress who played an idealistic young radio astronomer named Ellie Arroway in the film Contact. Tarter was not the model for the driven researcher at the center of Carl Sagan's book of the same name, although she understands why people often make that assumption. In fact, she herself did so after reading the page proofs that Sagan had sent her in 1985. After all, both she and Arroway were only children whose fathers encouraged their interest in science and who died when they were still young girls. And both staked their lives and careers on the search for extraterrestrial intelligence (SETI), no matter how long the odds of detecting an otherworldly sign. But no, Tarter says, the character is actually Sagan himself--they all just share the same passion.

In her position as director of the Center for SETI Research at the SETI Institute in Mountain View, Calif., Tarter has recently focused on developing new technology for observing radio signals from the universe. The concept, first presented in the 1950s, is that a technologically advanced civilization will leak radio signals. Some may even be transmitting purposefully.

Searching for Life in Our Solar System; The Search for Alien Life; Exclusive Online Issues; by Bruce M. Jakosky, sidebar by Richard Lipkin; 5 Page(s)

Since antiquity, human beings have imagined life spread far and wide in the universe. Only recently has science caught up, as we have come to understand the nature of life on Earth and the possibility that life exists elsewhere. Recent discoveries of planets orbiting other stars and of possible fossil evidence in Martian meteorites have gained considerable public acclaim. And the scientific case for life elsewhere has grown stronger during the past decade. There is now a sense that we are verging on the discovery of life on other planets.

To search for life in our solar system, we need to start at home. Because Earth is our only example of a planet endowed with life, we can use it to understand the conditions needed to spawn life elsewhere. As we define these conditions, though, we need to consider whether they are specific to life on Earth or general enough to apply anywhere.

Searching for Life on Other Planets; The Search for Alien Life; Exclusive Online Issues; by J. Roger P. Angel and Neville J. Woolf, sidebar by Corey S. Powell; 7 Page(s)

The possibility that we are not alone in the universe has fascinated people for centuries. In the 1600s Galileo Galilei peered into the night sky with his newly invented telescope, recognized mountains on the moon, and noted that other planets were spheres like Earth. About 60 years later other stargazers observed polar ice caps on Mars, as well as color variations on the planet's surface, which they believed to be vegetation changing with the seasons (the colors are now known to be the result of dust storms). During the latter part of this century, cameras on board unmanned spacecraft captured images from Mars of channels carved by long gone rivers, offering hope that life once may have existed there. But samples of Martian soil obtained in the 1970s by the Viking lander spacecraft lacked material evidence of any life. Indeed, the present conditions in the rest of our solar system seem to be generally incompatible with life like that found on Earth.

But our search for extraterrestrial life has recently been extended--we can now turn our attention to planets outside our own solar system. After years of looking, astronomers have turned up evidence of planets orbiting three distant stars similar to our sun [see box on pages 23 and 24]. Planets around these and other stars may have evolved living organisms. Finding extraterrestrial life may seem a Herculean task, but within the next decade, we could build the equipment needed to locate planets with life-forms like the primitive ones on Earth.

The Case for Relic Life on Mars; The Search for Alien Life; Exclusive Online Issues; by Everett K. Gibson, Jr., David S. McKay, Kathie Thomas-Keprta and Christopher S. Romanek; 7 Page(s)

Of all the scientific subjects that have seized the public psyche, few have held on as tightly as the idea of life on Mars. Starting not long after the invention of the telescope and continuing for a good part of the past three centuries, the subject has inspired innumerable studies, ranging from the scientific to the speculative. But common to them all was recognition of the fact that in our solar system, if a planet other than Earth harbors life, it is almost certainly Mars.

Interest in Martian life has tended to coincide with new discoveries about the mysterious red world. Historically, these discoveries have often occurred after one of the periodic close approaches between the two planets. Every 15 years, Mars comes within about 56 million kilometers of Earth (the next approach will occur in the summer of 2003). Typically, life on Mars was assumed to be as intelligent and sophisticated as that of Homo sapiens, if not more so. (Even less explicably, Martian beings have been popularly portrayed as green and diminutive.)